Add the ebus_mqtt target: transport interface, PublishHold, reconnect order

[dcj]

Adds ebus_mqtt, the first component of the split in #3: the Homie-agnostic MQTT layer, with no dependency and only its own include directory. ebus_core links it, and PlatformIO still builds one library.

What moved

From	To
Transport interface in homie/homie_transport.h	MqttTransport in mqtt/include/ebus/mqtt/transport.h; HomieTransport stays in homie/homie_transport.h as a subclass with the Homie adapter
ports/posix/include/ebus_posix/publish_hold.h, ports/posix/src/publish_hold.cpp	mqtt/include/ebus/mqtt/publish_hold.h, mqtt/src/publish_hold.cpp
ports/posix/test/test_publish_hold.cpp (hand-rolled checks)	test/test_mqtt_publish_hold/ (Unity under CTest)
Reconnect order inline in PahoTransport::try_connect()	mqtt_after_connect() in mqtt/include/ebus/mqtt/reconnect.h, mqtt/src/reconnect.cpp, with test/test_mqtt_reconnect/

Layout: each component gets <name>/include/ebus/<name>/ and <name>/src/ with its own CMakeLists.txt, so discovery/ and homie/ can follow the same shape. Naming stays namespace-free like the rest of the core, with Mqtt/mqtt_ prefixes and EBUS_MQTT_ macros.

Interface change

Before:

virtual bool queue_publish(const char* topic, const char* payload, int length,
                           bool retained, Property* source) = 0;   // HomieTransport

After:

typedef void (*mqtt_publish_done_fn)(void* ctx, const char* payload, int length, bool sent);

// MqttTransport (ebus_mqtt)
virtual bool queue_publish(const char* topic, const char* payload, int length, bool retained,
                           int qos, mqtt_publish_done_fn done, void* ctx) = 0;

// HomieTransport : MqttTransport (homie/homie_transport.h)
bool queue_publish(..., int qos, mqtt_publish_done_fn done, void* ctx) override;  // default: refuse, log, done(false)
virtual bool queue_publish(..., bool retained, Property* source);  // default: forward at homie_qos(retained)

Property still calls the Property* overload. Its default forwards to the generic one with a callback that calls Property::queued_publish_done(), so the publish-on-change memo works either way. A port overrides one of the two.

PublishHold no longer logs; hold() returns EVICTED_OLDEST / DROPPED_TOO_LARGE and the port logs. Sizes are EBUS_MQTT_HOLD_ENTRIES (64), EBUS_MQTT_HOLD_TOPIC_MAX (127), EBUS_MQTT_HOLD_PAYLOAD_MAX (1024).

esp32-sdk compatibility

MqttClientTransport overrides the Property* overload, which is still virtual, so esp32-sdk builds with no source change. Checked in a scratch worktree of esp32-sdk main with lib/ebus_core pointed at this branch (nothing committed there):

• ./ebus-esp32 build (esp32-poe-iso): builds with no warnings. firmware.bin 1,436,352 bytes on main (cpp-sdk v0.1.0), 1,436,480 with this branch, so +128 bytes (.flash.text +32, .flash.rodata +80, RAM sections unchanged).
• ./ebus-esp32 test: 11/11 suites pass, including the two new test_mqtt_* suites under PlatformIO's native env. That confirms library.json's -I mqtt/include reaches the library's dependents.

Tests

• Core (cmake -S . -B build, warnings as errors): 10/10 CTest suites pass with Apple Clang and GCC 16.
• POSIX port (-DEBUS_POSIX_WERROR=ON): 6/6 e2e tests pass against a local mosquitto on 127.0.0.1. The reconnect test checks the flush, re-subscribe, notify order from the log, and those log lines are unchanged.
• ebus_mqtt alone: every source and header compiled with -I mqtt/include only (Clang and GCC), plus add_subdirectory(cpp-sdk/mqtt) from a scratch project.
• New CI step "ebus_mqtt alone" in core-host-build.

Part of #3. Closes #4.

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